The explosive growth of the Internet has created a demand for high data rates for business and residential users (SOHO—small office/home office). Because of the prevalence of twisted pair copper wires in existing telephone networks, much of the demand must be met by data communication protocols that are adapted to transmit data over these standard analog plain old telephone systems (POTS) lines. The need for high-speed access to the home and businesses appears to be ever increasing due in part to the availability of information, data, high-bandwidth video and the like from the World Wide Web. Because of this ever increasing demand, higher speed modems are required.
Originally data transmission over POTS lines was accomplished using voice/data modems. These devices modulate data just like voice signals. As a result their theoretical data transfer speed limit is insufficient to deliver broadband content. Current voice/data modems operate at a maximum data transfer speed of up to 56.6K bits/second.
Due to the shortcomings of voice/data modems, the industry looked for new solutions for delivering high speed data access over existing twisted pair copper telephone lines. One result of these efforts was the emergence of digital subscriber line technology (xDSL). As used herein “xDSL” is understood to denote any type of DSL service (e.g., ADSL, DMT-based VDSL, VDSL2, etc.). DSL provides high speed data transmission over relative short distances of twisted pair lines by utilizing the portion of the available bandwidth in the twisted pair above the few thousand kilohertz utilized by voice communications. Thus, the available bandwidth is divided into three bands: the lowest frequency band allocated to voice communications, then, the next band allocated to upstream data transfer while the remaining higher bandwidth, the majority, is reserved for downstream data communications. Filters are used to prevent interference between bands. Within the upstream and downstream bands, the available bandwidth is further divided into harmonically related sub-carriers or tones approximately 4 KHz in width on which data is simultaneously transferred. Because of bandwidth limitation (4 KHz), and power limitation of the telephone network, line coding schemes are used to encode digital signals into analog signals that convey the analog information over the analog telephone network. The line coding schemes manipulate the analog carrier signal, which has three attributes, amplitude, phase and frequency. One or more of such attributes may be manipulated by known modulation techniques such as, for example, quadrature amplitude modulation (QAM) whereby the carrier signal's phase and amplitude are modulated to encode more data within a frequency bandwidth. One example of a QAM modulation system sends two bits of information per QAM symbol, where the digital values can be encoded and the corresponding amplitude and phase can be represented using a constellation. Increasing the constellation size, that is number of points (bits), will cause the bit density per symbol to increase, and hence achieve higher data rates.
An upper limit on this process of constellation mapping stems from the fact that as the constellation size increases, the granularity of the phase and the amplitude difference between different constellation points diminishes, making it increasingly difficult to decode the constellation points, especially in the presence of noise. One way of circumventing this problem is to increase the Euclidean distance between symbols by employing trellis coding. Trellis coding is particularly well suited for this because it is bandwidth efficient, since the symbol rate and required bandwidth is not increased. As noted above, as the constellation size gets bigger, the problem of detecting a constellation increases due to the greater symbol density. Therefore, a way of counter-acting the effects the short Euclidean distance between symbols is to partition the quadrature amplitude modulated signal into subsets, thereby creating an acceptable Euclidean distance between symbols.
A property of DSL-based systems is that system performance is directly correlated to loop length, that is, the distance of the channel between the transmitting modem and receiving modem. Unfortunately, the telephony loop introduces severe frequency-dependent attenuation of the signal. In frequency-division multiplexed (FDM) DMT, filters are required to separate upstream from downstream transmission. Hence, the impulse response length of the composite equivalent discrete channel depends not only on the cable characteristics, but also on the impulse responses of the transceiver filters and service splitter—plain old telephone service (POTS) or integrated services digital network (ISDN)—included in the end-to-end signal path. The nonzero impulse response length results in inter-DMT symbol interference. This can be avoided by inserting a cyclic prefix (a copy of the last samples of the DMT symbol) between DMT symbols. The length of the prefix (samples) must be longer than the memory of the channel. In doing so, orthogonality between the carriers of the same symbol is also restored. The transient at the beginning of each DMT symbol (introduced by transmission over the channel) is absorbed in the prefix, which is removed at the receiver. The outputs of the demodulating FFT then equal the transmitted QAM symbols multiplied by the channel transfer function taken at the carrier frequencies. Hence, channel equalization is easily performed by multiplying each FFT output with a single complex coefficient equal to the inverse of the channel transfer function at the corresponding frequency.
As noted above, loop attenuation due to inter-symbol interference is greatest at the highest frequency tones. As the loop gets longer, the loop attention at the higher frequencies will be much larger than at lower frequencies. When attenuation on a particular sub-carrier gets to be too great, that sub-carrier is effectively turned off. Therefore, the maximum usable bandwidth in a DSL system is a non-linear, monotonically decreasing function of loop length. In conventional DSL systems, approximately 3.5 miles or about 18,000 feet is the maximum loop length at which acceptable performance can be maintained. While cyclic extension and equalization may be used to allow longer loop lengths, the length of the extension must be greater as the loop length increase. Transmitting this redundant data decreases the effective data transfer rate. Also, because loop conditions may change, setting a particular extension length and sampling frequency fails to accommodate changes in loop conditions.